refactor: removing code for validating non null getAuthenticatedUser function

[himan085]

Discussion: #680

When PDK generates code for the frontend React website, it checks if the function getAuthenticatedUser is defined or not: code pointer
This function is fetched from the context and used inside useEffect as shown below:

const { getAuthenticatedUser } = useCognitoAuthContext();

  useEffect(() => {
    if (getAuthenticatedUser) {
      const authUser = getAuthenticatedUser();
      // code doing stuff using authUser
    }
  }, [getAuthenticatedUser, setUsername, setEmail]);

After this PR in aws-northstar this check is no longer required since the getAuthenticatedUser has a default state for the CognitoAuthContext which defines the getAuthenticatedUser method: code pointer

Removing the non-null check for getAuthenticatedUser function definition at all places in this PR.

[agdimech]

Thanks for contributing :)